sandbox: add missing compatible property in device tree

In the device tree UEFI unit test the compatible property of the device is
read.

Provide the missing property.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

arch/sandbox/dts/sandbox.dts

@@ -6,6 +6,7 @@
 	#address-cells = <1>;
 	#size-cells = <1>;
 	model = "sandbox";
+	compatible = "sandbox";
 
 	aliases {
 		i2c0 = &i2c_0;

arch/sandbox/dts/sandbox64.dts

@@ -6,6 +6,7 @@
 	#address-cells = <2>;
 	#size-cells = <2>;
 	model = "sandbox";
+	compatible = "sandbox";
 
 	aliases {
 		i2c0 = &i2c_0;